The present invention relates to the protection of active sites on a semiconductive wafer which is sawed into individual active-site-bearing chips, and, more particularly, to a method of and apparatus for protecting, during and after sawing, such active sites in a plurality of arrays or matrices from debris and deleterious substances which are produced by, or are used in, such sawing.
Numerous processes are known for producing plural arrays of active sites in and on a first surface of a semiconductor wafer. Each active site may comprise one or more transistors and may include an integrated circuit having other circuit components. The wafer is ultimately separated into a plurality of individual chips, also known as dies or bars, each of which includes one of the active site arrays, the array having a "top" surface comprising a portion of what was formerly the wafer's first surface. Each active site array has associated therewith one or more bond pads on its top surface. The bond pads are rendered selectively electrically continuous with the active sites, typically by depositing or otherwise forming them on top of, and in electrical contact with, conductors formed on the wafer. Some of the same steps used to produce the active sites may also produce the conductors, which are themselves electrically continuous with the active sites.
The separation of the wafer into individual chips is effected by an operation which may be referred to as "sawing." Sawing separates the wafer along lines or paths extending between locations whereat adjacent active site arrays reside or will ultimately reside.
Sawing, which typically involves mechanical abrasion and erosion of the wafer, may be achieved by a number of techniques, including those which utilize rotating saw blades and vibrating tips. Accordingly, the act of sawing the wafer itself produces substantial debris which includes small pieces of the wafer and possibly small pieces of the saw blade or vibrating tip. Sawing is also typically accompanied by cooling/lubricating fluids and other substances which prevent the saw blade or tip from damaging the wafer and which prolong the life of the saw blade or tip.
The debris resulting from and the substances used in sawing can degrade the performance of or render inoperative the active sites. As a consequence, wafers are often sawed into chips before carrying out the processing which produces the active sites. The resulting chips are maintained in a chip matrix after sawing, and the chip matrix is processed to produce the active site arrays on each one thereof.
If the active sites include a spatial light modulator ("SLM"), such as that known as a deflectable mirror device or a digital micromirror device (collectively "DMD"), each active site may be even more sensitive to the effects of the debris and fluids resulting from and used in sawing.
A DMD is a multlayered structure formed on a wafer, which includes a light-reflective beam or similar mechanical member. The member is associated with an active site and is so mounted to, or hinged from, the material of the wafer as to be deflectable or movable between a normal position and other positions. Deflection of the beam may be achieved by electrostatically attracting the beam toward (or to) an adjacent electrode which is at a different electrical potential from that of the beam. Deflection of the beam stores energy in its mount or hinge, which stored energy tends to return the beam to its normal position. Movement of the beam, which may be binary or analog, is controlled by the circuit components of the active site associated with the beam and functioning as an addressing circuit. Deflection of the beam is facilitated by an undercut well which underlies the beam. The well is formed by appropriate etching of one of the layers of material deposited on the wafer.
In use, an array or matrix of DMD's is arranged to receive light from a source. The received light which is incident on the reflective beams is selectively reflected or not reflected onto a viewing surface depending on the position of the beams. Such reflected light is directed by each beam onto the viewing surface in only one selected position, which may be the normal position or one of the other positions. In all other positions of each beam other than the selected position, the incident, reflected light is directed in such a way that it does not fall on the viewing surface. Appropriate energization of the circuit components of the addressing circuit associated with each beam of each active site in the array or matrix permits the beam-reflected light on the viewing surface to be presented as a rasterized array of pixels (as in a typical television) or as a scanning line of pixels (as in a line printer). Thus, the beam of each active site is or acts as a pixel.
Because a DMD includes circuit components as well as a microminiature deflectable beam, it is especially sensitive to debris resulting from sawing the wafer and to the fluids and other substances used to facilitate sawing. Such debris can enter the undercut well and prevent deflection of the beam. Thus, in one extant technique, formation of the circuit components of the active sites and etching or other steps which define the beams are followed by the deposit of a protective layer thereon. Sawing of the wafer to separate the arrays then proceeds, the protective layer preventing the sawing operation from damaging the circuit components and the etch-defined beams. After sawing is completed, the protective layer is removed and the undercut wells are then formed under each beam. Formation of the wells at this time obviates the sawing-related and substances from entering the wells.
Sawing of wafers before active site formation requires either that the resulting chips be accurately maintained in their original relative orientation during active site producing processing or that each chip be individually processed to produce its array of active sites. These expedients are costly and their implementation is time-consuming. The above-noted sequence, in which circuit component formation and beam-defining etching occur before sawing, nonetheless requires the emplacement of the protective layer, which is removed following sawing. The deposit of and subsequent removal of the protective layer, which serves only the purpose of protecting the circuit components and the etch-defined beam during sawing, is, again, costly and time-consuming.
One object of the present invention is the provision of a method of, and apparatus for, protecting each active site in plural active site arrays on a fully processed semiconductor wafer, particularly active sites which include a DMD SLM or other micromachine, which DMD, in turn, includes both circuit components and a deflectable, light-reflecting beam, so that circuit component formation, beam-defining etching and well formation may all be carried out on an unsawed wafer to thereby minimize processing steps while protecting the active sites from sawing-caused damage.